1. Field of the Invention
This invention relates to polarized light irradiation apparatus for optical alignment of liquid crystals in which an alignment layer of a liquid crystal display element is irradiated with polarized light.
2. Description of Related Art
Liquid crystal display elements are produced by carrying out an alignment process to align the liquid crystals in the desired direction on an alignment layer formed on the surface of a transparent substrate; two of these transparent substrates are put together with the alignment layers facing inwardly, such that alignment layers are separated by a gap of a desired size, and thereafter, liquid crystal is injected into the gap.
The alignment process for the alignment layer is a technology called optical alignment, in which the alignment layer is exposed by irradiation with polarized light of a particular wavelength.
The most widely used liquid crystal display elements, called TN liquid crystal, are constructed such that the direction of alignment of the liquid crystal is rotated 90xc2x0 between the two transparent substrates. Accordingly, it is necessary to have two transparent substrates with their alignment layers in different directions.
Moreover, the angle of view of the liquid crystal can be improved by dividing one pixel of the liquid crystal display element into two or more parts and changing the alignment direction of the liquid crystal in each of the divided pixels. This method is called the pixel division method or the multi-domain method.
When optical alignment is applied to this pixel division method, a mask is used and one portion of the divided pixel formed on the substrate is irradiated with polarized light, after which the mask is replaced and the other divided portion is irradiated with light with a different polarization direction.
It is desirable that the polarized light irradiation apparatus, used to irradiate the alignment layer with polarized light, be provided with the ability to change the desired direction of polarization of the polarized light to any desired direction for irradiation. The present applicant has proposed, in EP 1020739 A2, a light irradiation apparatus in which the direction of the polarized light used to irradiate the substrate can be changed without rotating the substrate by means of a rotating the polarization element within the light irradiation apparatus. An example of the structure of the light irradiation apparatus capable of changing the direction of the polarized light is illustrated in FIG. 11.
In FIG. 11, the light from the lamp 1 is condensed by an ellipsoidal condenser mirror 2, reflected by the first plane mirror 3 and is incident on the polarization element 8. The polarization element 8 is, for example, a device with multiple glass plates inclined at Brewster""s angle with respect to the optical axis; light of polarization P passes through but most light of polarization S is reflected. By this method, it is possible to obtain polarized light with the desired extinction ratio. The polarized light P that emerges from the polarization element 8 is incident on the integrator lens 4, then passes through the shutter 5 and is reflected by the second plane mirror 6 into the collimator lens 7. The resulting parallel light rays pass through the mask M and irradiate the alignment layer of the workpiece (substrate) W. In this apparatus, the polarization element 8 is free to rotate around the optical axis of the center of the light flux that is incident on the polarization element 8, and by rotating the polarization element 8, it is possible to change and set, as desired, the direction of the polarized light that irradiates the alignment layer.
In addition, there are apparatus that, instead of rotating the polarization element 8, rotates the substrate stage (not illustrated) on which is mounted the workpiece W substrate on which the alignment layer is formed.
In the light irradiation apparatus, the required length of the optical path is determined from opti-metric issues including the area irradiated, the distribution of intensity, and the degree to which the rays irradiating the substrate are parallel. In order to make the light irradiation apparatus smaller while still maintaining the same length of optical path, the optical path is folded using the first plane mirror 3 and the second plane mirror 6.
These plane mirrors are fabricated by the vapor deposition of aluminum or some other metal on a quartz plate. To prevent damage to the mirror, the surface of the mirror is often covered with a protection layer, normally from 10 nm to more than 100 nm in thickness. Examples of such protection layers are magnesium fluoride (MgF2), silicon dioxide (SiO2), or aluminum oxide (Al2O3).
In order to optically align the alignment layer, the polarized light must be at a certain wavelength and must have an extinction ratio at or above a certain level. These values are determined by the properties of the alignment layer. The extinction ratio is the ratio between polarization P component of the light and the polarization S component. An extinction ratio of 10:1 or better is desirable for aligning the alignment layer. And it is common to use ultraviolet light with a wavelength in the range of 250 nm to 350 nm. However, in the apparatus in FIG. 11, even if a nearly linearly polarized light (with a good extinction ratio, between 20:1 and 15:1, for example) is emitted from the polarization element, it sometimes happens that when the polarization element is rotated to change the direction of polarization of the polarized light irradiating the substrate, the extinction ratio drops (to 6:1, for example) and the desired extinction ratio cannot be obtained.
As an example, if the polarization element is glass set at Brewster""s angle, the polarized light P is a linearly polarized light. Further, it is known that generally, when linearly polarized light is reflected by a mirror, the phase shift of the reflected light can cause elliptical polarization. It is this elliptical polarization that is the cause of the extinction ratio reduction.
A simple explanation of the reason for elliptical polarization is shown in FIG. 12, which is an illustration showing the reflected state when a linearly polarized light emerges from the polarization element and is incident on the mirror with an angle of incidence of 45xc2x0. The polarized light that emerges from the polarization element has an electrical field that is aligned up and down with respect to the plane of the paper (the direction of polarization is up and down in the plane of the paper).
In FIG. 12, the optical axis of the polarized light that emerges from the polarization element is in the plane A determined by the direction of the electrical field of the polarized light. On the other hand, the optical axis of the light that is incident on the mirror (which is the light that emerges from the polarization element) is in plane B determined by the optical axis of the light reflected from the mirror. FIG. 12 illustrates the situation where plane A and plane B are parallel to each other. However, if the polarization element is rotated around the optical axis, plane A and plane B are no longer parallel, and the polarization element is inclined 90xc2x0 from its state in FIG. 12; plane A and plane B would then be perpendicular to each other.
At the plane of reflection of the mirror, the component of the light that is incident on the mirror that is parallel to plane B is the polarization component P, and the component that is perpendicular to plane B is the polarization component S.
When plane A and plane B are in a parallel or perpendicular relationship, the direction of the electric field of the polarized light incident on the mirror (the direction of polarization) can have only a polarization P component or a polarization S component. For example, if the direction of the electric field of the polarized light incident on the mirror (the direction of polarization) is as shown in FIG. 13, the light that is incident on the mirror will have both a polarization P component and a polarization S component.
There is generally a phase shift when light is reflected by a mirror, and that phase discrepancy is known to be different for the polarization P component and the polarization S component. When the polarized light incident on the mirror has only a polarization P component or a polarization S component (when plane A and plane B are in a parallel or perpendicular relationship), the phase shift still occurs, but the reflected light is a linearly polarized light with the same extinction ratio as the incident light, since there is only a polarization P component or a polarization S component.
However, if the polarized light is incident on the mirror having both a polarization P component and a polarization S component as in FIG. 13, then as described above, the phase shift of the polarization P component and the phase shift of the polarization S component will be different, causing a phase difference between the polarization P component and the polarization S component, resulting in an elliptically polarized light as shown in FIG. 14 (for greater detail please see, for example, The Applied Physics Handbook, ed. Applied Physics Society, Maruzen Co., pp. 20-22). That is, even with an ideal linearly polarized light (infinitely large extinction ratio), if plane A and plane B are not in a parallel or perpendicular relationship, the light emerging from the mirror will be an elliptically polarized light. The extinction ratio of the elliptically polarized light will be lowered, since it is expressed as the ratio of the minor axis of the ellipse to the major axis shown in FIG. 14.
In the apparatus shown in FIG. 11, therefore, when the polarization element 8 is rotated to change the direction of polarization of the polarized light that irradiates the workpiece W, the relationship between plane A and plane B will no longer be parallel or perpendicular; the polarized light becomes elliptical when reflected by the second plane mirror, and the extinction ratio of the polarized light that irradiates the workpiece W is thereby reduced.
The object of the instant invention is to solve the problem of the conventional irradiation technology described above. The invention described herein provides a polarized light irradiation apparatus that uses optical alignment to irradiate an alignment layer formed on a substrate with polarized light that is incident on a reflecting mirror and is reflected by that reflecting mirror such that changing the direction of polarization of the polarized light incident on the reflecting mirror does not reduce the extinction ratio of the polarized light irradiating the substrate, relative to the extinction ratio of the polarized light emerging from the polarization element.
Taking the bearing of the linearly polarized light incident on the mirror as xcex1 and the difference in phase shift between the Polarization P component and the Polarization S component due to reflection by the mirror as xcex94, then the angle of ellipticity xcex5 determined by the ratio of the major axis and the minor axis of the elliptically polarized light of reflected light has the relationship sin 2xcex5=sin 2xcex1xc3x97sin xcex94 (see, for example the xe2x80x9cHandbook of Optical Engineeringxe2x80x9d, Asakura Shoten, Jul. 25, 1981, page 412).
Moreover, the extinction ratio can be expressed as (1/tan xcex5)2:1. From this it can be seen that the smaller the angle of ellipticity, a higher extinction rate can be obtained. When a linearly polarized light with an extinction rate of infinity:1 is incident on the mirror, if xcex5=0, then the extinction rate of the reflected light will be infinity:1.
On the other hand, sin 2xcex5=sin 2xcex1xc3x97sin xcex94, and so if xcex94=0, then xcex5=0 as well, in which case the extinction ratio does not depend on the direction of polarization of the polarized light, and becomes infinity:1. As xcex94 becomes larger, xcex5 can increase as well. In that event, when the polarized light is incident on the mirror with the direction of polarization inclined at 45xc2x0, the extinction ratio of the reflected light will be 1:1.
Accordingly, in order not to reduce the extinction rate of the reflected light, the reflecting mirror should be constituted so that ideally xcex94=0, or xcex94 0.
Calculating from the equation above, when the polarized light is incident on the mirror with an extinction ratio of 15:1, obtaining a reflected light with an extinction ratio of 10:1 requires that xcex94xe2x89xa6xc2x120xc2x0, a reflected light with an extinction ratio of 12:1 requires that xcex94xe2x89xa6xc2x115xc2x0, and a reflected light with an extinction ratio of 13.5:1 requires that xcex94xc2x1≅10xc2x0.
As stated above, to align an alignment layer on a substrate it is desirable that the extinction ratio be at least 10:1. That requires that xcex94xe2x89xa6xc2x120xc2x0, and the means described below enables xcex94xe2x89xa6xc2x120xc2x0 to be achieved.
The difference xcex94 between phase shifts of the polarization P component and the polarization S component depends on the wavelength and angle of incidence of the light as it is incident on the mirror. In the event that a protection layer has been formed on the surface of the mirror, the type of layer and its thickness are factors as well. Therefore, as explained below, it is possible to set the angle of incidence of the polarized light arriving at the mirror and the thickness of the protection layer at appropriate values, and then position the mirror so as to hold down the difference xcex94 in the phase shifts of the polarization P component and the polarization S component.
(1) When a layer is formed on the surface of the mirror, and when light is incident on the mirror, the phase of the light that passes through the layer and is reflected from the mirror is delayed, in accordance with the optical thickness of the layer, relative to the phase of the light that is reflected from the surface of the layer.
By taking advantage of this phase shift due to formation of a layer, it is possible to reduce the difference xcex94 in the phase shifts of the polarization P component and the polarization S component.
This difference xcex94 in the phase shifts depends on the angle of incidence of the polarized light on the mirror. Further, the wavelength used for optical alignment is decided by the type of optical alignment layer. Accordingly, if the type of protection layer formed on the surface of the mirror and the optical thickness of that layer are selected in accordance with the wavelength needed to align the optical alignment layer and the angle of incidence of the polarized light on the mirror as decided by the optical design, it will be possible to make xcex94xe2x89xa6xc2x120xc2x0 and to make the extinction rate at least 10:1.
(2) If the angle of incidence of the polarized light on the reflecting mirror is small, it is possible to reduce the difference xcex94 in the phase shifts of the polarization P component and the polarization S component of the reflected light.
Consequently, if multiple reflecting mirrors are combined and the angle of incidence of the polarized light on each reflecting mirror is kept small, it is possible to make xcex94xe2x89xa6xc2x120xc2x0 and to make the extinction rate at least 10:1.
(3) Using first and second reflecting mirrors of the same material, by positioning them so that the plane determined by the optical axis of the light incident on the second reflecting mirror and the optical axis of the reflected light is perpendicular to the optical axis of the light incident on the first reflecting mirror and the optical axis of the reflected light, so that the angle of incidence of the first reflecting mirror is the same as the angle of incidence of the second reflecting mirror, then it is possible to have the phase shift caused by reflection in the first mirror negated by the reflection in the first mirror.
Consequently, by using at least two reflecting mirrors, the first and the second, and by positioning the reflecting mirrors as described above, the phase shift of the polarization P component and the polarization S component can be eliminated, and the polarized light reflected from the second reflecting mirror can be a linearly polarized light.